RP6: Functional group tests

Tests for alcohol, aldehyde, alkene and carboxylic acid.

Describe the test for alcohols.

In the test for alcohols, a small piece of sodium metal is added.
There will be effervescence as hydrogen gas is produced.

Describe the test to distinguish between primary, secondary, and tertiary alcohols.

In the test to distinguish between primary, secondary, and tertiary alcohols, 10 drops of the alcohols are added to 2cm³ of acidified potassium dichromate (VI) solution.
The mixture is warmed gently in a water bath.

Explain the colour changes for each type of alcohol when acidified potassium dichromate (VI) solution is added.

When acidified potassium dichromate (VI) solution is added to primary alcohols, the orange solution turns green as an aldehyde forms.
When added to secondary alcohols, the orange solution turns green as a ketone forms.
When added to tertiary alcohols, the orange solution stays orange.

Suggest why a further test is needed to distinguish between primary and secondary alcohols.

A further test is needed to distinguish between primary and secondary alcohols because the test with acidified potassium dichromate (VI) solution shows the same colour change.

Describe the test using Fehling’s solution.

In the test using Fehling’s solution, 2cm3 of Fehling’s solution is added to a test tube.
5 drops of the aldehyde or ketone are added to a test tube.
The test tube is placed in a boiling hot water bath.

State the colour change of Fehling’s solution when an aldehyde is added and when a ketone is added.

When an aldehyde is added to Fehling’s solution, the blue solution will give a brick-red precipitate.
When a ketone is added to Fehling’s solution, the blue solution will remain blue.

Describe how to make Tollens’ reagent.

To make Tollens’ reagent, add 2 cm³ of 0.10 mol dm^-3 silver nitrate solution to a test tube.
Add a few drops of dilute sodium hydroxide solution. This will form a light brown precipitate.
Add drops of dilute ammonia solution until the brown precipitate dissolves completely.

Describe the test using Tollens’ reagent.

In the test using Tollens’ reagent, the test tube containing Tollens’ reagent is added to a hot water bath.
10 drops of the aldehyde or ketone are added to the test tube.

State what is formed when an aldehyde is added to Tollens’ reagent and when a ketone is added to Tollens’ reagent.

When an aldehyde is added to Tollens’ reagent, a silver mirror forms on the walls of the test tube.
When a ketone is added to Tollens’ reagent, the solution remains colourless.

Suggest why the aldehyde/ketone is not heated over a flame.

The aldehyde/ketone is not heated over a flame because they are both very flammable.

Describe the test for alkenes.

In the test for alkenes, 2cm³ of the solution is added to a test tube.
2cm³ of the bromine water is added to the test tube.
The test tube is shaken.

Describe the colour change in the test for alkenes.

In the test for alkenes, the solution will go from orange to colourless.

Describe the test for carboxylic acids.

In the test for carboxylic acids, 2cm³ of the solution is added to a test tube.
Sodium carbonate is added to the solution.
The solution will effervesce and the gas produced will turn limewater cloudy.